Tachycardia is a type of arrhythmia which presents with a high heart rate, typically above 100 for an adult. The disorder results in a speeding of normal heart rhythm which is known as sinus tachycardia and which may be brought about by a number of factors, such as exercise, anaemia, fever, anxiety, pregnancy or drugs. Alternatively, sinus tachycardia may be caused by an underlying pathological condition as a result of an arrhythmia.
An electrocardiogram (ECG) is typically used to classify the type of arrhythmia. Tachycardias may be classified as either narrow complex tachycardias (supraventricular tachycardias) or wide complex tachycardias. Narrow and wide refer to the width of the QRS complex on the electrocardiogram (ECG). Narrow complex tachycardias tend to originate in the atria, while wide complex tachycardias tend to originate in the ventricles. Tachycardias can be further classified as either regular or irregular.
Ventricular tachycardia (VT or V-tach) is a potentially life-threatening cardiac arrhythmia that originates in the ventricles. It is usually a regular, wide complex tachycardia with a rate between 120 and 250 beats per minute. Ventricular tachycardia has the potential of degrading to the more serious ventricular fibrillation. Ventricular tachycardia is a common, and lethal, complication of a myocardial infarction (heart attack).
Supraventricular tachycardia is a type of tachycardia that originates from above the ventricles.
Examples of narrow complex tachyarrhythmias include: atrial fibrillation, atrial flutter, AV nodal reentrant tachycardia, accessory pathway mediated tachycardia, atrial tachycardia, multifocal atrial tachycardia and junctional tachycardia.
Atrial fibrillation is one of the most common cardiac arrhythmias. It is generally an irregular, narrow complex rhythm. However, it may show wide QRS complexes on the ECG if bundle branch block is present. It may be difficult to determine the rhythm's regularity when the rate exceeds 150 beats per minute. Depending on the patient's health and other variables such as medications taken for rate control, atrial fibrillation may cause heart rates that span from 50 to 200 beats per minute (or even higher if an accessory pathway is present). However, new onset atrial fibrillation tends to present with rates between 100 and 150 beats per minute.
AV nodal reentrant tachycardia (AVNRT) is the most common reentrant tachycardia. It is a regular narrow complex tachycardia that usually responds well to the Valsalva manoeuvre or the drug adenosine.
AV reentrant tachycardia (AVRT) requires an accessory pathway for its maintenance. AVRT may involve orthodromic conduction (where the impulse travels down the AV node to the ventricles and back up to the atria through the accessory pathway) or antidromic conduction (which the impulse travels down the accessory pathway and back up to the atria through the AV node).
Orthodromic conduction usually results in a narrow complex tachycardia, and antidromic conduction usually results in a wide complex tachycardia that often mimics ventricular tachycardia.
Junctional tachycardia is an automatic tachycardia originating in the AV junction. It tends to be a regular, narrow complex tachycardia and may be a sign of digitalis toxicity.
Tachycardias resulting from a fast heart rate tend either to be sinus tachycardia or an abnormal tachyarrhythmia, such as one which is supraventricular or ventriculuar in origin. The primary symptoms of sinus tachycardia may be perceived as palpitation. In susceptible individuals, this sensation can even induce anxiety. Typically, the symptoms of sinus tachycardia tend to be benign unless the patient has coexistent pathology which is worsened by a high heart rate, e.g. coronary ischaemia (angina), heart failure or heart valve disease. This can then lead to breathlessness or chest pain or in rare circumstances myocardial infarction, or acute on chronic heart failure. Tachyarrythmias may cause dizziness, fainting and black outs etc.
There is therefore a need for effective heart rate lowering agents for use in the treatment of tachycardia disorders.